~&crobial mats are conspicuous benthic communities composed almost entirely of prokaryotic microorganisms. These mats develop in extreme environments which limit the growth of Eukaryotic organisms. Nficrobial mats are found in high temperature, high salinity, high alkalinity, and high sulfide environments where microorganisms can accrete due to the absence of predatory organisms. In spite of the extreme conditions, these systems have well developed biogeochernical cycles and are extremely biologically active. The systems are generally driven by photosynthetic organisms, such as cyanobacteria. which fix C02 and N2, and do not require external input of organic matter. A complex community of both aerobic and anaerobic bacteria develop around the primary production of the cyanobacteria. Some of the constituent members of the microbial community are metabolically versatile, utilizing a variety of electron donors and electron acceptors. ~&crobial mats have been implicated in promoting the rapid degradation of crude oil, most likely due to the overall metabolic diversity of the community. This study focuses on hypersaline marine microbial mats from Solar Lake, Egypt. The Solar Lake microbial mats have been characterized by extensive biological, chemical and physical examination. During the diumal cycle, the surface layers of the mat undergo significant changes in physical and chemical conditions. These transient conditions include changing oxygen concentrations (from 0 to > 1000 micromolar), sulfide concentrations (0 to > 500 micromolar), pH (from neutral to >9.5), temperature, organic carbon (in quantity and type), among others. Tbe high photosynthetic activity of the cyanobacteria creates a situation during the day in which the surface layers (0-2.5 mm) are oxygenated, often in excess of saturation. However, during the night, diffusion from the overlying water column is the only source of oxygen, and the system becomes anaerobic at a depth of 0.5 mm. Oxygen is consumed at all times by a variety of heterotrophic and lithotrophic organisms, causing rapid depletion of oxygen below regions of photosynthetic production or rapid diffusion. Below these oxygenated regions, sulfide concentrations, due to the activity of the sulfate reducing bacteria (SRB), increase rapidly. Nficrobial populations may respond to these environmental changes by altering their metabolism or by relocating to more favorable position in the mat. Nficrocoleus chtonoplastes, predominant microbial mat cyanobacteria, have been observed to undergo migratory behavior in response to changing light conditions. These bacteria produce large sheath tubes (identified in previous microscopy) which penetrate the mat vertically, and they may migrate inside these sheaths. Theoretically the sheaths may also be utilized by other bacteria, although previous microscopy did not identify other organisms inside the sheaths. One of the main elements of this research is to link mat physical structure with mat function, and in particular, migration of mat microorganisms and transport of pollutants. One aspect of this is to characterize the physical and biology'structure of the microbial mat matrix without modification due to fixing. Previous microscopy of the mat employed fixing procedures using gluteraldehyde, as well as ethanol and propylene oxide for dehydration. We are interested in employing rapid freezing techniques to preserve our sample with a minimum of alteration. Of particular concern is preservation of the polysaccharide matrix of the mat. The mat has a thick and rubbery texture (especially the surface layer) due to the copious production of polysaccharides by mat bacteria. Overall, we would be most interested in examine the structure of the top 4 mm of the mat. In addition to being the most biologically active region of the mat, we also have a significant amount of population data for this region, at extremely high resolution (50 microme-ter vertical sections.) We are interested in looking at the overall structure and at the dimensions of the pore spaces in the mat matrix. Open spaces, in addition to the sheath tubes, may serve as conduits for migration of mat microbiota and for transport of pollutants. Specifically, we would like to look at the mat matrix in two dimensions: vertically, to look a the change in texture with depth, and horizontally, to assess heterogeneity in the mat matrix at a single depth.